The invention relates to an arrangement and a method for controlling an automatic shift device of a gear-change transmission of a motor vehicle.
German Patent document DE 3,214,710 C3 discloses a device for generating an electrical output signal in a control unit of a motor vehicle. The electrical output signal occurs during downshifting within the forward gears or into the reverse gear of a transmission when the manual shift lever is moved in the direction of the new gear and the driving speed in the forward direction of travel is still too high. The driving speed is recorded by an electronic speedometer. In order to afford sufficient protection against inadmissible down-shifts, to be easily adaptable to different backshift devices, and to be achieved, as required, at a very low additional outlay, provision is made in the known device for the control unit to have a logical switching stage and for an output signal to occur at the output of the logical switching stage. The output signal is for the actuation of a warning and/or blocking means. This occurs only when a first input signal derived from the driving speed is present at the first input of the logical switching stage before a second input signal which, derived from the shift-lever actuation, is present at the second input. The German Patent document DE 3,214,710 C3 indicates that the use of this known device in conjunction with a blocking means is also expedient in an automatic transmission.
German Patent document DE 4,120,566 A1 discloses a method for controlling a preferably electrohydraulically actuated automatic-shift transmission of a motor vehicle equipped particularly with an internal-combustion engine. The internal-combustion engine is capable of being influenced by a power-control member, preferably an accelerator pedal or a throttle flap. Gear steps of the transmission are shifted automatically via shift characteristic diagrams at least in dependence on the throttle-flap position, on the driving speed and on the engine speed. Upshifts (reduction in the transmission ratio) are prevented as long as the upshift-prevention state is active. The upshift-prevention state changes to the active state when a time change in the throttle-flap position falls below a negative limit value and the overrun mode is recognized. The upshift-prevention state changes to the inactive state after the expiry of a first timespan when the traction mode is recognized.
In order to improve the shift behavior before bends in the road and during braking, provision is made, in the known method, in the event that the overrun mode is recognized again during the expiry of the first timespan, for the upshift prevention to remain in the active state until the traction mode is recognized once more and a second timespan has elapsed. In this known method, provision is also made, preferably as a result of the active upshift prevention, for stepped downshifts to become possible. However, the stepped downshift takes place only when:
1) a service brake of the motor vehicle is actuated, or, alternatively thereto or in addition, the time change in the driving speed is lower than a first negative longitudinal-acceleration limit value, PA1 2) the transverse acceleration recorded by the transverse-acceleration sensor is below a first transverse-acceleration limit line, PA1 3) the time change in the driving speed is higher than a second negative longitudinal-acceleration limit value, and PA1 4) the driving speed is lower than a driving-speed limit value.
The downshift takes place by one gear step in each case, at least one fourth timespan being located between two shifts. The stepped downshift is carried out up to that gear step which is admissible at the instantaneous operating point of the motor vehicle in the instantaneously set shift characteristic diagram.
Vehicles with automatic transmissions have, as a rule, a selector lever with a gate shift. In addition to the four main driving steps P, R, N and D, the shift gates conventionally have further gear steps which make it possible to block gears. Consequently, downshifts can be initiated by the driver and upshifting can be prevented. However, the possibility of manual action also entails the risk of faulty operation. Overspeeds can occur as a result of downshifts in the upper engine-speed range. This results in damage to the engine.
Faulty operations can be prevented by mechanically/hydraulically blocking the selector lever against downshifting in inadmissible driving-speed or engine-speed ranges. This is a solution which is complicated and sensitive to tolerances. In the case of electronically controlled transmissions, manual selector-lever downshifting could be allowed, but downshifting cannot be executed in the inadmissible engine-speed range. Downshifting then takes place only when the engine speed falls below the maximum downshift engine speed or driving speed at which engine overspeeding no longer occurs. The disadvantage of this solution is that the driver could not or could only poorly comprehend the shift behavior of the transmission. The transmission does not shift when shifting is initiated. At a decreasing speed, the transmission shifts down very abruptly at its highest engine-speed limit in a completely unmotivated way at a later time. Since the output torque and therefore the wheel force are increased by the amount of the gear jump of the multi-step transmission as a result of the downshift, this can lead to dangerous driving states in critical driving situations, such as, for example, on road bends or when the road surface is slippery.
There is therefore needed, for electronically controlled automatic transmissions, a method which allows the downshift block to function in a simple manner and which, at the same time, rules out the disadvantages mentioned above.
These needs are met according to the present invention by an arrangement and a method for controlling an automatic shift device of a gear-change transmission of a motor vehicle. A selector device is connected to the shift device by a manual selection member. An electronic control unit transmits a gear-shift signal to the shift device in dependence on a stored shift-line characteristic diagram in each case, by the processing of input signals which contain significant information relating to the respective position of the manual selection member, the transmission output speed, the engine load and the current gear. A first device checks whether the position of the manual selection member limits the upshift into a gear which is lower than the current gear. A second device forms a reference signal either identical to the position of the manual selection member or identical to the current gear. A third device checks whether the reference signal corresponds to a gear which is lower than the current gear. A storage stage contains at least one stored shift-line characteristic diagram. A computing stage checks whether, at the current driving speed, after a gear shift the engine speed would reach a value higher than a maximum value. A change-over stage can be changed over between a first switching state, in which its input is connected to one of its two outputs and a second switching state, in which its input is connected to its other output. At least one shift stage triggers the gear-shift signal. One output of the first device for checking the position of the manual selection member is connected both to a control input for setting the reference signal to the position of the manual selection member of the second device for forming the reference signal and to a control input of the change-over stage for changing over to the first switching state and to one input of the storage stage. Another output of the first device for checking the position of the manual selection member is connected to one input of the third device for checking the reference signal. One output of the third device for checking the reference signal and one output of the computing stage as well as the output of the storage stage are connected to a shift stage, if appropriate in each case to a shift stage. Another output of the third device for checking the reference signal is connected to the input of the change-over stage. The output of the change-over stage which is connected to the input of the change-over stage in the first switching state is connected to one input of the computing stage. The other output of the change-over stage is connected to the input of the storage stage. Another output of the computing stage is connected both to a control input of the change-over stage for changing over the latter into the second switching state and to a control input of the second device for forming the reference signal. The control input is used to set the reference signal to the value of the current gear.
According to the present invention, an engine, a gear-change transmission with electronic control, and a selector lever, the individual shift positions of which are sensed electrically, are used. If appropriate, the sensing of the shift positions can also take place in the transmission on a notched plate. The electronic transmission control has inputs for the selector-lever signal, the engine speed and the transmission output speed and/or the driving speed. Provided in the electronic transmission control is a shift program which, on the basis of the operating conditions, evaluates a shift-line characteristic diagram and which thereby executes automatic upshifts and downshifts in dependence on various parameters. By means of this selector-lever signal, upshifts can be suppressed and downshifts initiated.
In the method according to the present invention, after the engine speed or driving speed has fallen below its maximum admissible limit value, downshifting does not take place, but instead the evaluation of the shift-line characteristic diagram is initiated. As a result, no unmotivated downshift cab occur, but the transmission shifts down in dependence on the driving conditions according to the predetermined shift points. Consequently, after an inadmissible selector-lever downshift, there is no possibility that the vehicle can assume dangerous states in critical driving situations.
The essential advantages of the present invention are described in the following.
The driver would like to initiate a downshift in the transmission via the selector lever. If the speed of the vehicle is higher than the maximum admissible speed of the gear to be selected, this downshift would lead to inadmissibly high engine speeds and consequently to damage to the engine and/or transmission. According to the present invention, therefore, this shift is blocked. In this state, the selector-lever position and the selected gear step are no longer correctly related. The selector lever indicates a lower gear step than that selected in the transmission. The method according to the invention ensures that the correct relationship between the selector lever and transmission is restored so that the vehicle does not encounter any critical driving situation. For this purpose, according to the invention, a downshift may take place only at the downshift line at which the gear selected by the selector lever is entered, since the moment from which downshifting can be allowed again and the moment when the selector lever is actuated can be markedly different from one another.
This method differs from the state of the art, since, in the known instance, although a selector-lever shift is likewise prevented, as described above, nevertheless the downshift into the blocked gear is already allowed again when the driving speed falls below a limit at which the engine reaches its maximum admissible speed. This can lead to dangerous driving situations, in which the driver loses control of his vehicle.
In the known method of German Patent document DE 4,120,566 A1 described above, downshifts are stopped when a rapid easing of the accelerator pedal and additional criteria, such as, for example, transverse acceleration, are recognized. These are downshifts which, without these criteria, would be permitted according to the set shift characteristic diagram. They are stopped in order to maintain the driving stability of the vehicle. The downshifts are permitted again when the following criteria are satisfied: 1) when the brake is actuated or negative acceleration is recognized, 2) when the transverse acceleration falls below an appropriate value, 3) when the vehicle speed is lower than an appropriate value, and 4) when positive acceleration is recognized.
A downshift by only one gear step is permitted in each case. If, according to the shift characteristic diagram, downshifts by more than one gear step are allowed, a time meter is started. Only after the expiry of the time meter can a further downshift be initiated.
In contrast to this, according to the present invention, a selector-lever downshift leading to inadmissibly high engine speeds is prevented. Shifts via the shift characteristic diagram are not permissible in this range. They are permitted again only in dependence on the shift characteristic diagram, not on additional other criteria, such as transverse acceleration, brake actuation or the like. If, according to the shift characteristic diagram, downshifts over one or more gears are permissible, these are executed immediately and without the use of corresponding time meters. The time criterion does not in any way also have to be processed in the present invention, it would make it impossible for the driver to comprehend that downshifting is permitted again.
The invention consequently differs from the state of the art both in respect of the initiating criterion and the processed signals for permitting downshifting again, and also in respect of the shift sequence of the downshifts permitted again.
It is an advantage of the present invention to check and, if appropriate, permit a downshift by more than one gear instead of stepped downshifts, when the position of the manual selector lever necessitates per se such a downshift in relation to the current gear.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.